The two most common species of tree squirrels on the East Coast, and probably throughout the U.S., are the gray squirrel and the fox squirrel. Both are scatter hoarders: They do not physically defend their food, but rather divide and hide it. They have adapted to solve what Jacobs — now a professor of psychology and neuroscience at the University of California, Berkeley — has dubbed “the password problem”: “You just set up a new login, and you need a password that you can remember but no one else can crack.” A nut left in the open will go the way of Jacobs’ Easter eggs; yet a nut too well hidden will be lost even to the hoarder. The more densely a squirrel distributes its nuts, the easier it becomes for a wily neighbor to steal them all.
Years after her Easter trauma, Jacobs began studying squirrels as a graduate student in Princeton’s biology department. Little was known about the squirrel’s storage decisions when she started observing them in the Institute Woods. In the decades since, Jacobs has steadily expanded our understanding of one of North America’s most recognizable mammals.
Squirrels must consider the energy expense and exposure to predation involved in hiding, monitoring, and retrieving each nut. Somewhat counterintuitively, they often cache in open fields, where exposure to avian predation is greatest. Jacobs interprets this risky behavior as a strategy to leverage their precise knowledge of the nut’s location. “This makes it very hard for someone else to go out there and use trial and error, random search, to look for a cache,” she said.
The process of choosing a specific hiding spot within these spatial chunks is not simply a squirrel reflex; it involves careful analysis. “They pick up the nut, and then they very carefully assess it,” said Jacobs. “Its weight; its species; its size.” They also consider the density of nuts and other scavengers in the area, among other factors. Jacobs is now collaborating with a mathematician to model these tradeoffs. “It seems like there’s going to be a very interesting optimization algorithm,” she said.
As no primates aside from humans store food, squirrels might offer one of the best analogs for human storage behavior. “I wouldn’t be surprised if there are economic truths that we can find in squirrels that can be applied to humans that wouldn’t necessarily occur to an economist,” Jacobs said.
Her research could also yield applications in human health. Jacobs has observed that the male gray squirrel’s hippocampus — and specifically the CA1 subfield, which is critical to spatial memory and navigation, and which is one of the first parts of the brain to deteriorate in human Alzheimer’s patients — grows to peak size in the fall. This is unusual for a summer-breeding rodent. Jacobs believes that this growth may be due to the cognitive exercise associated with caching, and that similar activities might boost hippocampal plasticity in humans. “Is there a caching task that humans could use that might protect their CA1 from degeneration?” she wonders.
The Jacobs Lab recently received a new grant to set up what Jacobs calls “squirrel schools” — nurseries for orphaned squirrels — which it will use to study the development of spatial cognition, economic decision-making, and motor learning. “It’s very hard to study in the adult, where it’s already fully developed,” Jacobs said. “But if you look where it’s not present, then it becomes present, then it’s full-blown, you get an idea of the mechanism.”
Jacobs accidentally began her first squirrel school while a graduate student at Princeton. People would bring orphaned and injured squirrels to the biology department, which would forward them to its resident squirrel expert. “I had an office in Eno Hall, and it was full of squirrels,” she recalled. “I didn’t have a license to rehabilitate, but things were a little more lax in those days.”
Jacobs credits her success to one of her mentors, former Princeton professor Henry Horn, who died last month. “My whole career has been inspired by him,” said Jacobs, who plans to join many of her fellow ecology and evolutionary biology alumni at Horn’s memorial service in the University Chapel on May 5. “He was an incredible, astute, brilliant biologist.”